1. Field of the Invention
The present invention relates to an objective lens and a method of manufacturing an optical pickup apparatus.
2. Description of the Related Art
<<Designing and Manufacturing of Lens>>
Lens Design Principle
An objective lens is known, for example, as a plastic molded lens or a glass molded lens mounted on an optical pickup apparatus, for focusing light emitted from a light source on an information surface of an optical disc. FIG. 15A is a top plan view of the objective lens 500 viewed from the side of the lens surface L2 thereof with a small curvature side the optical disc (from O side of the line O-O′ of FIG. 5B), FIG. 15B is a sectional view of the objective lens 500 taken along the line A-A′, and FIG. 15C is a top plan view of the objective lens 500 viewed from the side of the lens surface L1 thereof with a large curvature side the light source (from O′ side of the line O-O′ of FIG. 5B).
The lens surface L1 has, in an outer region thereof, a surface to be positioned on the light-source side (hereinafter, referred to as a source-side edge surface S), of an annular plane portion (hereinafter, referred to as an edge) having a thickness and having a normal direction that is substantially the same as the direction of the optical axis of the lens surface L1. The lens surface L2 has, in an outer region thereof, a surface to be positioned on the optical-disc side (hereinafter, referred to as a disc-side edge surface S′) of the edge. The lens surface L2 is concaved relative to the disc-side edge surface S′, so that the lens surface L2 is protected, for example, when the objective lens 500 being disposed on the disc-side edge surface S′.
The conventional objective lens 500 is generally designed such that the source-side edge surface S to be positioned on the lens surface L1 side is used as a mounting reference surface. For example, the tilt or the decentering of the lens surfaces L1 and L2 is adjusted by a lens molding die so that the aberration is within specification when incident light strikes at right angles with respect to the objective lens 500 on the source-side edge surface S. That is, the shape of the lens molding die is adjusted so that the optical axes of the lens surface L1 and L2 are orthogonal to the source-side edge surface S and so that the optical axis of the lens surface L1 coincide with the optical axis of the lens surface L2. No restrictions are generally imposed on the tilt of the disc-side edge surface S′ relative to the source-side edge surface S, so that the tilt is formed as finished (see, e.g., Japanese Patent Application Laid-open Publication No. 8-075597 described below).
Lens Molding
Assume, for example, that five different types of objective lenses 500a to 500e have been formed as shown in FIGS. 16A to 16E, respectively, as a result of lens molding under the lens design principle described above.
The objective lens 500a of type A shown in FIG. 16A has the lens surfaces L1 and L2 whose optical axes are orthogonal to the source-side edge surface S and coincide with each other. The disc-side edge surface S′ is parallel to the source-side edge surface S. In this case, one optical axis of the whole lens can be defined. This optical axis is hereinafter referred to as a lens optical axis X.
The objective lens 500b of type B shown in FIG. 16B has the lens surfaces L1 and L2 whose optical axes are orthogonal to the source-side edge surface S and coincide with each other. The disc-side edge surface S′ is tilted with respect to the source-side edge surface S. In this case, the lens optical axis X can be defined.
The objective lens 500c of type C shown in FIG. 16C has the lens surfaces L1 and L2 whose optical axes coincide with each other and are orthogonal to the disc-side edge surface S′. The disc-side edge surface S′ is tilted with respect to the source-side edge surface S. In this case, the lens optical axis X can be defined.
The objective lens 500d of type D shown in FIG. 16D has the lens surface L2 whose optical axis is orthogonal to the source-side edge surface S and the lens surface 1 whose optical axis is tilted with respect to the source-side edge surface S. The disc-side edge surface S′ is parallel to the source-side edge surface S. In this case, on optical axis of the whole lens cannot be defined.
The objective lens 500e of type E shown in FIG. 16E has the lens surface L1 whose optical axis is orthogonal to the source-side edge surface S and the lens surface 2 whose optical axis is orthogonal to the disc-side edge surface S′. The optical axis of the lens surface L2 and the disc-side edge surface S′ is tilted with respect to the source-side edge surface S. In this case, one optical axis of the whole lens cannot be defined.
Lens Inspection and Shipping
FIG. 17 depicts a configuration of a conventional lens inspection device 200. The lens inspection device 200 includes an inspection light source 210, an inspection lens holder 220, an inspection lens 230, a mirror 240, an interferometer 250, and a personal computer 260.
Since the source-side edge surface S of the objective lens 500 is used as the mounting reference surface as the lens design principle, a lens receiving surface P of the inspection lens holder 220 is arranged to confront the source-side edge surface S of the objective lens 500. An adjustment is made in advance so that the lens receiving surface P is orthogonal to an optical axis Z of incident light from the inspection light source 210 to the objective lens 500.
In the lens inspection device 200, inspection light to be applied from the inspection light source 210 to the lens surface L1 of the objective lens 500 housed in the inspection lens holder 220. As a result, the converged inspection light from the lens surface L2 of the objective lens 500 is converted via the inspection lens 230 into parallel light, an optical path of which in turn is changed in the direction by the mirror 240 to enter the interferometer 250 connected communicably to the personal computer. The interferometer 250 is e.g., a Fizeau or Twyman-Green type interferometer, etc. capable of measuring wavefront aberration and causes incident light to interfere with reference light to generate interference fringes. The personal computer 260 provides a monitor display of whether the aberrations (astigmatism, coma, spherical aberration, etc.) determined by calculation based on information of interference fringes measured by the interferometer 250 are within specification. Thus, a person who performs the lens inspection can select the objective lens 500 conforming to the lens design principle by checking the monitor display of the personal computer 260.
As the result of the inspection of the five different types of lenses shown in FIGS. 16A to 16E, the objective lens 500a of type A (see FIG. 16A) and the objective lens 500b of type B (see FIG. 16B) are selected as shown in FIG. 18. That is, the objective lenses 500a and 500b are selected as conforming to the lens design principle described above, the lens optical axes X thereof capable of being defined, on the premise that the optical axes of the lens surfaces L1 and L2 are orthogonal to the source-side edge surface S and coincide with each other.
<<Optical Pickup Apparatus Assembly Process>>
Referring to FIG. 19, in a conventional assembly process of the optical pickup apparatus mounted with the objective lens 500a or the 500b selected in the lens inspection process described above, an optical block assembly step (S190), a rising mirror reflected light optical axis tilt adjustment step (S191), an actuator assembly step (S192), an actuator mounting step (S193), an actuator tilt adjustment step (S194), and an actuator tilt readjustment step (S195) are performed in sequence. The steps will hereinafter be described.
Optical Block Assembly Step (S190)
The optical block 300 is assembled, as shown in FIG. 20, by a holder 321 fitted with a semiconductor laser element 320 (hereinafter, abbreviated to LD), a diffraction grating 330, a beam splitter 340, a collimator lens 350, a rising mirror 360, and a sensor lens, a photodetector, etc. that are not shown, disposed on a metal or plastic housing 310.
The LD 320 is an element emitting laser light with a predetermined wavelength in response to a control voltage applied from a laser driving circuit (not shown). The diffraction grating 330 is an element diffracting laser light from the LD 320. Laser light diffracted by the diffraction grating 330 passes through the beam splitter 340 to enter the collimator lens 350. The collimator lens 350 is a lens converting laser light into parallel light. The rising mirror 360 is a mirror reflecting parallel light from the collimator lens 350 to allow the light to enter the objective lens 500 (not shown).
Rising Mirror Reflected Light Optical Axis Tilt Adjustment Step (S191)
The LD 320 of the optical block 300 is activated and the position adjustment of the LD 320 in the optical block 300 is performed as follows so that the optical axis Z of the reflected light from the rising mirror 360 becomes orthogonal to a reference surface Q.
First, as shown in FIG. 21, a tilt adjustment of shafts 370a and 370b to which the optical block 300 is fitted in a subsequent step, is performed by using an optical axis adjusting device 400. The tilt adjustment of the shafts 370a and 370b is performed by rotationally adjusting a biaxial goniostage 410 so that an image measured by an autocollimator 420 for a parallel-plate mirror 440 placed on the shafts 370a and 370b lies at an origin point of a monitor 430. As a result, the reference surface Q defined by a surface of the parallel-plate mirror 440 is positioned at an origin point of the autocollimator 420.
Next, as shown in FIG. 22, the optical block 300 is fitted to the shafts 370a and 370b, with pins of an LD position adjusting jig 450 being pressed against an aperture of the LD holder 321 shown in FIG. 20. The LD 320 is then activated and a stage mounted with the LD position adjusting jig 450 is adjusted so that the image of the autocollimator 420 lies at the origin point of the monitor 430, thus the position of the LD 320 in the optical block 300 being adjusted. After the position adjustment of the LD 320, the LD holder 321 is adhesively fixed to the housing 310. As a result, the optical axis Z of reflected light from the rising mirror 360 of light emitted from the LD 320 is set to become orthogonal to the reference surface Q.
Actuator Assembly Step (S192)
An actuator 600 is assembled, as shown in FIGS. 23A to 23C, which has at least a lens holder holding the objective lens 500 such that the edge surface S thereof is disposed on the lens receiving surface and that drives the objective lens 500. FIG. 23A is a top plan view of the actuator 600 viewed from a focusing direction orthogonal to both a tangential direction and a radial direction, FIG. 23B is a side view of the actuator 600 viewed from the radial direction, and FIG. 23C is a sectional view of the actuator 600 viewed from the radial direction. As used herein, the tangential direction is a direction orthogonal to the optical axis X of the objective lens 500 and a direction of a tangent of information tracks formed concentrically or spirally with a rotation center as a base point on an optical disc (not shown). The radial direction is a direction orthogonal to the optical axis X of the objective lens 500 and a direction of a radius of the optical disc. The focusing direction is a direction parallel to the optical axis X of the objective lens 500.
The actuator 600 has at least the lens holder 630 holding the objective lens 500, magnets 650a and 650b, tracking coils 632a to 632d, a focusing coil 634, suspension wires 640a to 640d, a suspension holder 660, and an actuator substrate 670, on a yoke base 610.
The lens holder 630 is in the shape of a rectangular box with a bottom opened and holds the objective lens 500 arranged on the upper surface side thereof by means of fixing, fitting, etc. It can be, e.g., a bobbin, etc. in the shape of a circular or polygonal cylinder forming a coil with electric wires wound.
The lens holder 630 is secured by the suspension wires 640a to 640d via the suspension holder 660 to the actuator substrate 670 by means of solders 672. That is, the lens holder 630 is resiliently retained on the actuator substrate 670 by resilient force of the suspension wires 640a to 640d. The lens holder 630 is driven in a tracking direction and a focusing direction by magnetic actions of the magnets 650a, 650b, etc., as a result of driving of the focusing coil 634 and the tracking coils 632a to 632d. 
The yoke base 610 is provided with three skew screw holes 620a to 620c to be used for mounting the yoke base 610 on the optical block 300. The yoke base 610 is further provided with a spherical seat 675 for adjusting the tilt of the actuator 600 with respect to the optical block 300.
Actuator Mounting Step (S193)
As shown in FIG. 24, the yoke base 610 of the actuator 600 is screwed to the housing 310 of the optical block 300 by using three skew screws 625a to 625c. By disposing a spring between the housing 310 and a thread, the skew screw 625a received in the skew screw hole 620a provides a resilient support for the actuator 600 on the housing 310, and can be a fulcrum for the tilt adjustment of the actuator 600. The skew screw 625b not shown received in the skew screw hole 620b serves to adjust the tilt of the actuator 600 in the radial direction, and the skew screw 625a received in the skew screw hole 620c serves to adjust the tilt of the actuator 600 in the tangential direction.
Actuator Tilt Adjustment Step (S194)
In the state where the actuator 600 is simply screwed to the housing 310 in the mounting step (S193) of the optical block 300, the lens optical axis X of the objective lens 500 cannot necessarily be orthogonal to the reference surface Q. If the tilt of the lens optical axis X relative to the orthogonal direction of the reference surface Q increases, then the aberration becomes large, resulting in deteriorated quality of a focus error signal or a tracking error signal, making it infeasible to provide servo control as well as to observe jitter corresponding to a RF signal.
Thus, as shown in FIG. 25, by fastening the skew screws 625b to 625c based on a display of the monitor 430 of the autocollimator 420, an adjustment can be made for reducing as much as possible the tilt of the lens optical axis X relative to the orthogonal direction of the reference surface Q. More specifically, by means of the optical axis adjusting device 400 shown in FIG. 21, the tilt of the surface defined by the shafts 370a and 370b is so set initially as to correspond to the origin point of the monitor 430 of the autocollimator 420. The optical block 300 screwed to the actuator 600 is then fitted to the shafts 370a and 370b so that the laser light from the autocollimator 420 is applied to the disc-side edge surface S′ of the objective lens 500, consequently checking the tilt of reflected light from the disc-side edge surface S′. The tilt of the actuator 600 is then adjusted by turning the skew screws 625b and 625c with a screwdriver 460 so that an image of the reflected light lies at the origin point of the monitor 430. As a result, the lens optical axis X of the objective lens 500 becomes substantially orthogonal to the reference surface Q.
If the autocollimator 420 is able to measure the tilt of the source-side edge surface S in the actuator tilt adjustment step (S194), then the tilt of the actuator 600 can be adjusted so that the source-side edge surface S and the reference surface Q become parallel to each other. However, the source-side edge surface S is in contact with the lens receiving surface P of the lens holder 220 and does not appear on the surface of the lens holder 220, with the result that light from the autocollimator 420 cannot be applied to the source-side edge surface S.
Even though part of the lens receiving surface P of the lens holder 220 is provided with a through-hole to allow light from the autocollimator 420 to enter from the lens surface L1 side, the presence of the rising mirror 360 will interfere with the measurement. Accordingly, merely making the tilt adjustment of the actuator 600 based on the disc-side edge surface S′ may result in a coarse adjustment to roughly correct the great tilt when the actuator 600 is mounted on the optical block 300, which will necessitate a next actuator tilt readjustment step (S195).
Actuator Tilt Readjustment Step (S195)
After the completion of the tilt adjustment step (S194) of the actuator 600, as shown in FIG. 26, the optical pickup apparatus in the form of the actuator 600 fitted with the optical block 300 is mounted with a photodetector (not shown). A spindle motor is then rotated with an optical disc 700 to activate the LD 320. At that time, the position of the photodetector (not shown), etc., is adjusted so that the level or the symmetry of the focus error signal is within specification.
Afterward, a focus servo control is provided to focus a spot on the information surface of the optical disc 700, thereby enabling the tracking error signal to be observed. The position of the photodetector (not shown) is then finally adjusted so that the symmetry of the tracking error signal is within specification. Subsequently, a tracking servo control is provided to cause the spot to follow the tracks of the optical disc 700, thereby enabling the jitter corresponding to the RF signal to be observed. If the jitter is within specification, then the actuator 600 including the photodetector (not shown) is adhesively fixed to the housing 310. If the jitter is out of specification, then the skew screws 625b and 625c are adjusted to try to reduce the jitter. As a result, if the jitter is still out of specification, the optical pickup apparatus is handled as a defective item.
The jitter is measured as a jitter value by means of a jitter meter (not shown). Therefore, it is difficult to directly know which one of the skew screws 625b and 625c is to be turned to which direction, with the result that a jitter bottom is to be found out by observing how the jitter value fluctuates by turning one of the skew screws 625B and 625c to a small extent, which is an extremely troublesome operation. Therefore a method of adjusting the skew screws 625b and 625c is proposed, which is observing a spot itself of a main beam of outgoing light of the objective lens 500 by a microscope.
For example, when the optical axis X of the objective lens 500 is parallel to the incident light optical axis Z, as shown in FIG. 27A, a spot is a circle or an ellipse having point symmetry. On the contrary, when the optical axis X of the objective lens 500 tilts with respect to the incident light optical axis Z, a coma will occur and hence, as shown in FIG. 27B, the spot comes to have a large side lobe in the direction of tilt of the lens. Thus, by checking the state of the side lobe, it is possible to find which one of the skew screws 625b and 625c is to be turned to which direction.
Up until now, the objective lens 500 has been designed and manufactured using the source-side edge surface S thereof as the mounting reference surface in conformity with the lens design principle described above. Accordingly, the objective lens 500 shipped through the lens inspection from a lens manufacturer becomes either the objective lens 500a of type A (see FIG. 16A) or the objective lens 500b of type B (see FIG. 16B). The result of the tilt adjustment of the actuator 600 will hereinafter be described separating the case of the objective lens 500a of type A and the case of the objective lens 500b of type B. In the following symbols, “∥” is representative of being parallel, “⊥” is representative of being orthogonal, and “≠” is representative of being not parallel.
In the case of the objective lens 500a of type A, the disc-side edge surface S′ and the source-side edge surface S are parallel to each other, with the lens optical axis X being orthogonal to the source-side edge surface S. By performing the rising mirror reflected light optical axis tilt adjustment step (S191), the rising mirror reflected light optical axis Z becomes orthogonal to the reference surface Q, while by performing the tilt adjustment step (S194) of the actuator 600, the disc-side edge surface S′ becomes parallel to the reference surface Q. As a result, as shown in FIG. 28, the lens optical axis X and the rising mirror reflected light optical axis Z become parallel to each other, whereupon the aberration of outgoing light from the objective lens 500a can be within specification. Since the autocollimator 420 cannot measure aberration, the objective lens 500a of type A with a small aberration proceeds intactly to the next actuator tilt readjustment step (S195). In the actuator tilt readjustment step (S195), the optical pickup apparatus mounted with the objective lens 500a of type A has an initial jitter value falling within specification and is selected as a non-defective item without being subjected to the skew readjustment.                i) Objective lens of type A→disc-side edge surface S′ ∥ source-side edge surface S        ii) Lens design principle→lens optical axis X ⊥ source side edge surface S        iii) LD position adjustment→rising mirror reflected light optical axis Z ⊥ reference surface Q        iv) Actuator tilt adjustment→disc-side edge surface S′ ∥ reference surface Q        ∴ Lens optical axis X ∥ rising mirror reflected light optical axis Z        
On the other hand, in the case of the objective lens 500b of type B, the disc-side surface S′ and the source-side edge surface S are not parallel to each other, with the lens optical axis X being orthogonal to the source-side edge surface S. By performing the rising mirror reflected light optical axis tilt adjustment step (S191), the rising mirror reflected light optical axis Z becomes orthogonal to the reference surface Q, while by performing the tilt adjustment step (S194) of the actuator 600, the disc-side edge surface S′ becomes parallel to the reference surface Q. As a result, the source-side edge surface S tilts relative to the reference surface Q, and the lens optical axis X does not become orthogonal to the reference surface Q. That is, as shown in FIG. 29, the lens optical axis X of the objective lens 500 does not become parallel to the rising mirror reflected light optical axis Z, resulting in a tilted state.                i) Objective lens of type B→disc-side edge surface S′≠source-side edge surface S        ii) Lens design principle→lens optical axis X ⊥ source-side edge surface S        iii) LD position adjustment→rising mirror reflected light optical axis Z ⊥ reference surface Q        iv) Actuator tilt adjustment→disc-side edge surface S′ ∥ reference surface Q        ∴ Lens optical axis X≠rising mirror reflected light optical axis Z        
Thus, in the case of the objective lens 500b of type B, performing only the tilt adjustment step (S194) of the actuator 600 by the autocollimator 420 results in the lens optical axis X uncoincident with the rising mirror reflected light optical axis Z, allowing the occurrence of an aberration which is out of specification. Sine the autocollimator 420 cannot measure aberration, the objective lens 500b of type B with a large aberration proceeds intactly to the next actuator tilt readjustment step (S195). In the actuator tilt readjustment step (S195), the optical pickup apparatus mounted with the objective lens 500b of type B has an initial jitter value which is out of specification, resulting in a need to perform the skew readjustment.
In this manner, even though the objective lens 500 conforms to the lens design principle, due to no restrictions imposed on the tilt of the disc-side edge surface S′ relative to the source-side edge surface S, the objective lens 500a of type A having the source-side edge surface S and the disc-side edge surface S′ that are parallel to each other or the objective lens 500b of type B having the source-side edge surface S and the disc-side edge surface S′ that are not parallel to each other, may be shipped.
Thus, in the actuator tilt adjustment step (step S194) of the assembly process of the optical pickup apparatus mounted with the objective lens 500a or the objective lens 500b, the tilt of the actuator 600 is adjusted using the disc-side edge surface S′ since the tilt of the source-side edge surface S cannot be measured by the autocollimator 420. At that time, in the case of the objective lens 500b of type B, the actuator tilt readjustment step (S195) has to necessarily be performed, as in the case of the objective lens 500a of type A, taking it into consideration that the lens optical axis X and the rising mirror reflected light optical axis Z cannot be parallel to each other. In this case, the actuator tilt readjustment step (S195) results in a troublesome step of the skew adjustment based on the observation of the jitter value or of the spot shape, thereby increasing a manufacturing cost of the optical pickup apparatus.